Insights on Climate-Driven Fluctuations of Cave <sup>222</sup>Rn and CO<sub>2</sub> Concentrations Using Statistical and Wavelet Analyses
Concepción Pla, Ángel Fernández‐Cortés, Soledad Cuezva, Juan José Galiana-Merino, Juan Carlos Cañaveras, Sergio Sánchez Moral, David Benavente
Abstract
Understanding the fluctuations in cave air concentrations and their climatic control is substantial not only to preserve the quality of indoor atmospheres but also to avoid the risk related to the presence of hazardous substances. In this study, we investigated the most influential factors affecting 222 Rn and CO 2 concentrations, the nature of their dynamics, and their coupling with climatic variations. For this purpose, we combined a set of mathematical methods that included a statistical and wavelet analysis of a 6-year time series in Rull Cave (Spain). Generally, the 222 Rn and CO 2 dynamic in cave air showed similar patterns. However, the obtained results show that these gases have a different frequency response. Thus, the annual component of 222 Rn and CO 2 is controlled by the relationship between external and internal temperatures. At low frequencies, both gases are affected by the same variables when the cave atmosphere reaches a minimum concentration. However, when the cave atmosphere is isolated from the outdoors, 222 Rn and CO 2 behave differently and disturbance caused by the visitors is evidenced in terms of the CO 2 concentration; the latter observation was confirmed by the wavelet analysis at high frequencies. In contrast, the 222 Rn concentration shows important variations following rainfall, which was weakly identified in the CO 2 concentration.